Generally, a core for a solid golf ball is made from a rubber composition. It is known that the conventional rubber composition includes a base rubber, a zinc salt of acrylic acid or methacrylic acid as a co-crosslinking agent of the base rubber, and organic peroxide as a crosslinking initiator.
The solid core made from such a rubber composition has an excellent resilience and flight performance. However, the rubber composition including zinc (meth)acrylate (i.e., zinc acrylate or zinc methacrylate) is not easily released from a metal mold. Accordingly, it is difficult to improve the productivity of the golf ball. The poor mold releasing property may be improved by mixing a fluorine-type or silicon-type releasing agent with the rubber composition. Alternatively, a surface treatment such as chrome plating and fluororesin coating is applied to the mold surface to improve the releasing operation between the mold surface and the rubber composition.
However, in the case that the rubber composition contains such a releasing agent, the vulcanization of the rubber composition may be softer as the content of the releasing agent increases. To compensate for the problem, it is necessary to add a curing agent to the rubber composition. As a result, the content of the base rubber to the rubber composition decreases, causing a poor durability of the obtained rubber.
On the other hand, in the case that the mold surface is treated to improve the releasing operation, the rubber composition does not have the above-mentioned problem about the former case, since the rubber composition itself is not affected by such a surface treatment. However, if a releasing agent which is liable to adhere is used for the surface treatment, a further washing step is needed to remove the adhered releasing agent from the inner surface of the mold before a following step. The additional washing step hinders the improvement of productivity of golf ball. On the other hand, if a releasing agent which is unlikely to adhere is used for the surface treatment, the washing step is not necessary. Such a releasing agent, however, is expensive, thereby raising the production costs of golf ball.
In addition, instead of using the releasing agent, magnesium (meth)acrylate having a better mold releasing property than zinc (meth)acrylate may be used as a co-crosslinking agent. However, the rubber composition containing magnesium (meth)acrylate provides less resilience and thereby worse flight performance to the obtained golf ball than that containing zinc (meth)acrylate.